Tracking the location of an individual or an object that can move in unknown directions has been a concern for a number of years. This has particularly been so for designers of urban communications systems. There are a large number of applications which require knowledge of the location of the object or person, such as fleet management, felons subjected to "house arrest", tracking of patients in health care units, the location of stolen or lost vehicles, the general location of any individual, etc. These systems have employed a variety of techniques including radio location, dead-reckoning and proximity systems.
In one type of system termed "Automatic Vehicle Location" (AVL), various systems are employed to determine the location of a vehicle for the purpose of monitoring the location of a fleet or to find a lost or stolen vehicle. The systems have utilized a number of methods, one of which is the radio location method. In the radio location method, a number of towers are disposed at predetermined locations around a city to allow communication with vehicles in their vicinity. The location method typically utilizes triangulation which requires a comparison of a received signal from the vehicle with respect to a time base to determine the time of arrival of the signal at the receiving antennas. Typically, these antennas have very high powered transmitters that are operable to turn on the transmitter in the vehicle from a remote location by transmitting a request for ID to the vehicle and that vehicle, having the corresponding ID, then transmitting back an answer. It is the receipt of this answer that allows the triangulation method to be facilitated. However, this type of system suffers from a number of disadvantages. First, only very few vehicles can be located in this manner and it is desired to determine the movements of large fleets of vehicles with this method. Second, they require a dedicated system, which requires a specific request for an ID and the assumption that the vehicle is within the vicinity of the transmitter. Therefore, any one locale would be limited to the number of transmitters and/or receivers that are disposed at that one locale. AVL Systems are described in Steven Ritter and Jan McCoy, "Automatic Vehicle Location--An Overview", IEEE Transactions on Vehicular Technology, Vol. VT-26, No. 1, February 1977, which is incorporated herein by reference.
In another type of system, conventional location systems such as Loran-C and GPS Systems are utilized which are satellite-based systems. These are very sophisticated systems and have been typically utilized by large fleet management systems. These systems are typically expensive, but provide for fairly accurate location of a vehicle in substantially real time. Of course, these systems require some type of satellite receiver and obtain their accuracy by relying upon the very solid time base that is provided by the satellite.
In another type of system, a person's location is determined by their proximity to a receiver, or their lack of proximity thereto. In health care units, transmitters are attached to an individual and when the individual leaves a pre-defined zone, a receiver fails to pick up the transmitted signal and sounds an alarm. This type of system is also utilized with house arrest programs for convicted felons that are not considered to be dangerous and are allowed to exist outside penal institutions. Other systems require the felons to check in at random times and to interface a coded bracelet with the receiver on the telephone to confirm that they are, in fact, the individual that has answered the phone. These sometimes can be combined with video systems.
Once the location of an individual or an object is determined, this information is then useful in controlling a paging system for selective delivery of messages. In some paging systems, the location of a paging receiver in a paging system is defined in a database for one of a plurality of hubs. In this manner, throughput can be increased by transmitting different messages simultaneously to two different hubs and two different defined sets of users. Of course, this assumes that a paging receiver resides in the hub having the ID of the pager in its database. However, one problem in the past has been the mobility of the paging receivers which allows an individual to travel between various hubs. The only way to update the database at present is for the user to call in to a central station and have the paging terminal database updated with the new location. This, of course, needs to be done on a central level.